Change/replace Company name in Teams free

PabloCoca 

You may change your organization name in "Teams" free account. But it's not as straightforward as expected.

First, you must find out your admin account name automatically created by Microsoft.

If you registered in Teams with your personal email address - name@domain.com , then your admin account name will be like admin@namedomain.onmicrosoft.com.

To find it out, go to the Teams app, and within any channel click on "Files" tab. From actions bar click on "Sync" button, to sync your files with OneDrive. On click, OneDrive's login windows will pop-up, with your admin account typed-in. If you click on sign-in button you will be asked to enter the password (which you don't know). From there click on password recovery option. As you choose to recover (reset) password, an email will be sent on your personal email account. Using the provided link in an email you will be able to set a password for your new admin@namedomain.onmicrosoft.com account.

Now using that account login to portal.azure.com
From the left top corner click on the burger menu and then click on "Azure Active Directory" link.

Then inside of Active Directory click on the "Properties" link from the left list.
Now, here you may set your new tenant name and Save it.

That's it. It may take a couple of minutes before it will be updated.

PabloCoca 

You can't rename a Teams Free org or even an Office 365 tenant.  But what you can do is create a new team with the correct name, invite the members to the new team and delete the old one.

Here's a writeup on how to delete a Free Team org:  https://support.office.com/en-us/article/delete-your-teams-free-organization-2c28cfcf-3fbf-497c-938f-8815a8f6a6b9